Neodymium-Iron-Boron (Nd—Fe—B) alloy magnets have generally been the permanent magnets with the highest available performance. Accordingly, Nd—Fe—B magnets are used in a number of applications, such as MRI and computer-related applications. Demand for Nd—Fe—B magnets has been continuously increasing, in particular from green energy applications, such as electric vehicles and gearless wind turbines. For these applications, the magnets may need to work at high temperatures, which is currently a weak point of Nd—Fe—B magnets. Nd—Fe—B magnets have a low Curie temperature (˜312° C.) compared with other permanent magnets, such as Alnico and Sm—Co magnets. The magnetic performance of Nd—Fe—B magnets may decay rapidly with increasing temperature. Therefore, for high temperature applications, the remanence and coercivity may be important properties.
For anisotropic Nd—Fe—B magnets, which are the magnets used for many high-performance applications, remanence can be enhanced by improving the alignment of the hard magnetic Nd2Fe14B grains. There are different approaches to increase the coercivity of Nd—Fe—B magnets. One method is to substitute Dysprosium (Dy) or Terbium (Tb) for Nd in the magnets, since (Dy,Tb)2Fe14B has a much higher anisotropy field than Nd2Fe14B. However, this coercivity enhancement may come at the expense of decreased saturation magnetization. To make the magnet work stably at 200° C., 10 wt. % Dy may be added into the magnet, which causes a significant decrease in remanence and (BH)max. In addition, Dy and Tb are much less abundant in the earth compared to the light rare earth elements, such as Nd and Pr. The heavy rare earth (HRE) elements (e.g., Dy and Tb) are the least abundant of the rare earth (RE) elements.
Recently, alternative approaches have been developed to decrease the use of Dy/Tb in sintered Nd—Fe—B magnets for high temperature applications, including the double alloy method and the grain boundary diffusion method. The aim of both methods is to form a shell of heavy rare earth rich R2Fe14B phase on the surface of the hard magnetic grains. The increased anisotropy field in the shell prevents the nucleation of reversed domains when the magnet is exposed to an external demagnetizing field. Despite the fact that the Dy/Tb content can be decreased by nearly 50%, Dy or Tb is still needed in these magnets.